Remembering History: A rescue helicopter surveys damage to the Pentagon as firefighters battle flames after an airplane crashed into the U.S. military Headquarters outside of Washington in an apparent terrorist attack, September 11, 2001. (REUTERS/Larry Downing LSD/HB)

A rescue helicopter surveys damage to the Pentagon as firefighters battle flames after an airplane crashed into the U.S. military Headquarters outside of Washington in an apparent terrorist attack, September 11, 2001. (REUTERS/Larry Downing LSD/HB)

A rescue helicopter surveys damage to the Pentagon as firefighters battle flames after an airplane crashed into the U.S. military Headquarters outside of Washington in an apparent terrorist attack, September 11, 2001. (REUTERS/Larry Downing LSD/HB)